MCA is the abbreviation of "“Maximum Credible Accident”, also referred to as design basis accident. It is a term for the most severe accident for which reactor safety systems must be designed. In such a case the safety systems must ensure that radiation exposure outside the plant does not exceed the accident limit values of the Radiation Protection Ordinance.

Accidents going beyond this – such as the Chernobyl accident – are frequently qualified in the German media as “super MCA”.

In the case of a pressurised water reactor, the MCA would e. g. be a breaking of the primary coolant pipe with subsequent superheating of the core. The MCA approach which was used in the first years of using nuclear energy – in particular in the USA. Later on, it was replaced by the concept of design basis accidents – also in the Federal Republic of Germany. The modern approach comprises the problem specifications of a broad spectrum of design basis accidents, including, among others, a multitude of different leakages and transients – such as incidents triggered by failure of energy or feed water supply – as well as impacts such as earthquakes, explosion pressure waves and air crashes. This approach leads to a more balanced safety concept.

The seriousness of a real accident in a nuclear plant is evaluated according to the internationally agreed scale (INES scale).

The accident happened in unit 4 of the Chernobyl reactor during shutdown of the reactor for revision purposes, i. e. a regular slow shutdown of the reactor to carry out maintenance and test measures. An additional test programme to inspect various safety features of the plant was planned to be carried out in this shutdown process. The objective of this test was to prove that the plant could also be controlled in case of a loss of coolant (loss-of-coolant-accident) and a failure of energy supply (emergency power situation) assumed to happen at the same time. In case of such an accident the reactor is immediately shut down. The mechanical energy of the rotor in the turbine-generator-set coming to a stop must then be sufficient to transitionally provide the power for the main feed pumps until the supply of the emergency cooling pumps by the emergency diesel generators will be ensured. This test was considered to be a mere conventional test in the area of electrical engineering, where no reaction to the nuclear part of the plant was expected. Due to deficiencies of the test programme, unexpected conditions during the performance of the test, several violations of operating regulations, and especially the unfavourable reactor-physical and safety-related features of this reactor type, a prompt supercritical power excursion occurred, i. e. a sudden power increase which was immediately out of control . It led to a rapid increase in energy release in the fuel elements and thus to the destruction of the reactor core. The heat stored in the fuel was transferred very quickly to the surrounding coolant and led to its spontaneous evaporation. Due to the high pressure build-up, the reactor exploded and the reactor building including its roof was destroyed and a multitude of fires were caused. Altogether it has to be pointed out that the concurrence of basic design deficiencies of the plant and mistakes and violations with regard to plant management are the causes of the reactor disaster.

As a result of the reactor disaster of Chernobyl the question has been raised if consequences for the nuclear power plants operated in Germany had to be drawn. In first statements, the Federal Research Minister at that time, Heinz Riesenhuber, said that the German reactors were “absolutely secure”. The Federal Minister of the Interior (BMI) competent for nuclear safety at that time gave order to the Reactor Safety Commission (RSK) to carry out an analysis and evaluation of the accident with respect to German nuclear power plants. In November 1986, the RSK stated with regard to the transferability of the accident to German plants "that a prompt critical power excursion as had happened in Chernobyl was excluded in a light-water reactor of German construction due to the inherent features and the technical design and that the safety concept of nuclear power plants in the Federal Republic of Germany was not put into question by the accident in Chernobyl".

The probability of another super MCA occurring in one of the 443 nuclear power plants operated world-wide is relatively small. However, it is not zero. And the risk is not hypothetical but real. That also applies to the German nuclear power plants, whose security is without doubt above the average security level of all plants, but here, too, the probability of a major event is not zero.

In the former German Democratic Republic and in the Federal Republic of Germany it was recognised that the provision of data to evaluate the situation must be systematised. The measurement network for the surveillance of environmental radioactivity was massively expanded in the following years and decision-making was structured essentially. Today the “Integrated Measurement and Information System for the Surveillance of Environmental Radioactivity” (IMIS) is continuously operated with expenses amounting to about 9 million Euro annually. It compiles all measured values and information from the Federal Government and the Laender and forms the basis for a target-oriented and transparent information policy with clear recommendations in case of emergency issued by BMU or, if they are only of regional importance, by the Land.

The situation in case of an inland nuclear accident with local release of radioactivity is evaluated by the respective Laender authority competent for disaster control. In case of an accident with large-area consequences for several Laender and in case of accidents abroad the situation is evaluated by the Federal Government (BMU) according to the Precautionary Radiation Protection Act of December 1986. The necessary measured data are collected jointly by the Federal Government and the Laender.

AAs in other cases, too, the Laender are competentfor all disaster control measures in case of an accident in a nuclear power plant. The Laender also operate measurement networks in the vicinity of the plants. The administrative districts, regional councils or independent cities are immediately competent for protection measures. They are supported on demand by the Federal Office for Civil Protection and Disaster Assistance (BBK, www.bbk.bund.de).If radioactivity plays a role in an emergency the Laender are also assisted by the Federal Office for Radiation Protection on demand. The Federal Government surveys environmental radioactivity over wide areas and all over the country, evaluates the data and can e. g. pronounce bans and restrictions for the consumption of food and use of feedstuffs. In co-operation with the Laender the Federal Government can recommend certain behaviour to the affected population. The legal basis is provided by the Precautionary Radiation Protection Act.

In the case of events in the area of a Land which have exclusively local consequences, the Land can make recommendations to the population.

The Laender disaster control authorities regularly train emergency measures in the vicinity of nuclear plants. Several times a year, the federal measurement networks are put into so-called intensive operation. About once a year the Laender authorities involved in the Integrated Measurement and Information System participate as well. Once every two years the co-operation of local, Laender and federal authorities with Germany’s neighbours, the EU and international organisations is trained under the co-ordination of international organisations such as the IAEA..

Within one hour, the federal Integrated Measurement and Information System can take up intensive operation.. The measured values are then made available online to all competent authorities via an Internet portal.

The reviews of the situation are continuously published on the BfS website ->Ionising Radiation ->Environmental Radioactivity (IMIS) (www.bfs.de/ion/imis) and on the BMU website. In case of emergency the reviews of the situation are continuously updated. One component of this offer of information is a current map of the situation representing the current gamma dose rate all over Germany. In case of an inland accident the information of the competent Laender authority are of primary importance within the framework of disaster control to control the situation in a radius of 25 km.

In case of emergency, among others, the thyroid gland can be exposed due to the uptake of radioactive iodine via inhaled air and food. If the body is oversupplied with non-radioactive iodine in the form of tablets the iodine uptake of the thyroid gland is blocked, i. e.the radioactive iodine is practically not taken up, which essentially reduces radiation exposure. This applies especially to children up to 18 years, since the iodine metabolism of the thyroid gland is at maximum at this age. In pregnant women the uptake of iodine tablets serves to block also the iodine uptake of the unborn child.

Potassium iodide tablets are distributed within a radius of 100 km of a nuclear power plant. The tablets for the area up to a distance of 25 km were given to the competent Laenderwho either pre-distribute them to the households according to the respective Laender plans or store them locally. Outside the 25-km radius BfS co-ordinates the distribution in case of emergency for the Federal Government and the Laender.

Under the impression of the Chernobyl accident the German Commission on Radiological Protection changed its recommendations for intervention levels for the distribution of iodine tablets. Thus, in case of emergency, measures can now become necessary up to a distance of 100 km instead of 25 km previously.

No, iodine tablets only make sense where large amounts of short-lived radioiodine are produced by nuclear fission, namely in nuclear reactors. In a vicinity of up to 25 km around a nuclear power plant the Laender are competent for the distribution of potassium iodide tablets. The tablets are either pre-distributed to the households or stored locally and only distributed in case of emergency, depending on the plans of the respective Land. BfS is competent for the distribution at distances from 25 to 100 km around nuclear power plants.

Within a radius of up to 25 km around a nuclear power plant the Laender are competent for the distribution of potassium iodide tablets. The tablets are either pre-distributed to the households or stored locally and only distributed in case of emergency, depending on the plans of the respective Land.

For distances of 25 to 100 km, BfS co-ordinates the storage and distribution in case of emergency. The tablets are stored at 8 sites all over Germany. In case of emergency the Federal Government delivers the tablets by road or by air to the Laender. The Laender accept them at certain delivery points and organise the further distribution to the places where they are distributed to the population. The objective of planning is that in case of emergency the iodine tablets reach every single citizen within 12 hours, irrespective of the time of day and the weather.

The iodine tablets only have their optimum effect if they are distributed and have been taken before the passing of the cloud. For the population near a plant that could mean that a pre-distribution to the households is a more practicable solution. However, in that case it must be ensured that all households participate and have the tablets ready at hand in case of emergency. For the area at a distance of 25 to 100 km to nuclear plants the targeted and quick distribution in the currently jeopardised areas downwind is the more efficient strategy.

A multitude of radioactive substances was released into the atmosphere. One has to distinguish noble gases, volatile substances, less volatile substances and transuranium elements. Among the noble gases are krypton and xenon isotopes (i. a. Kr-85, Xe-133), among the volatile substances are iodine, antimony, tellurium and caesium (i. a. I-131, Sb-125, Te-132, Cs-134, cs-137), among the less easily volatilised substances are zirconium, ruthenium, barium, cerium as well as strontium (Zr-95, Ru-103, Ru-106, Ba-140, Ce-141, Ce-144, Sr-89, Sr-90) and among the likewise not easily volatilised transuranium elements are plutonium, americium and curium isotopes (i. a. Pu-238, Pu-239, Pu-240, Am-241, Cm-242).

The half-lives of these radioisotopes differ considerably. They are between about 3 days for tellurium-132 and about 24,000 years for plutonium-239.

The half-live is the time interval in which half of the nuclei of a radioactive nuclide has decayed. With the same number of radioactive nuclei, short half-lives lead to a high radiation activity and long half-lives include a low radiation activity. After ten half-lives the amount is still 1/2¹⁰ = 1/1024, i. e. about a thousandth of the initial amount.

Of the radioactive substances released during the Chernobyl accident only the caesium isotope 137 can be measured in Germany. As it is a volatile substance, larger amounts of caesium were released and transported over far distances. Due to the half-live of about 30 years not even 40 % of this radioisotope have decayed so far.

At first one has to differentiate between direct early damage in employees of the Chernobyl reactor and firemen and late damage especially in the population. With regard to late damage one has to differentiate between reports on observed cases of illness and fatalities in the vicinity of Chernobyl on the one hand and, on the other hand, extrapolations of cases to be expected altogether.

According to the 2005 report of the Chernobyl Forum, an international committee of experts from United Nations organisations, about 50 fatalities are directly attributed to radiation effects resulting from the reactor accident. 3 persons immediately died because of serious injuries and burns. 28 persons out of the 134 employees and firemen with acute radiation syndrome died a few days or weeks after the accident (maximum time of survival 96 days). In addition to this, some persons received high skin doses up to 500 Gy due to beta radiation, which caused serious burns and posed additional problems to medical treatment. The bone marrow transplantations carried out in 13 patients with acute radiation syndrome proved to be little successful. Only two of the patients treated in such a way survived. In the following years (1987 to 2004), further 19 persons died who had been treated because of an acute radiation syndrome, 3 of them with serious blood diseases. No acute radiation damage was reported of the population, in particular of the evacuated persons from the vicinity of Chernobyl.

With regard to the observed cases of illness in the general population in the vicinity of the reactor, there has been no proof of an increase in leukaemia cases so far but a clearly enhanced number of thyroid gland tumours, especially in children and youths, has been observed as well as an almost doubled breast cancer risk in women in the highly exposed regions in Belarus and the Ukraine. While the thyroid gland tumours already occurred a relatively short time after the accident, the increase in the breast cancer rate started about 10 years after the accident. An increase in the number of case rates for other solid tumours is to be expected for the future.

For the liquidators for whom doses above 150 mGy have been documented, a 2.2-fold higher rate of leukaemia incidences was determined for the first 10 years after the accident (1986 – 1996), compared with a group with lower exposures. For the second period of investigation from 1997 to 2003, this difference in the leukaemia incidence rates could not be confirmed any more. An increase in cardiovascular diseases became apparent for liquidators who had received a dose of more than 150 mGy in less than 6 weeks within the scope of their activities in the 30-km-zone.

Risk assessments known from other populations (Japanese atomic bomb survivors, persons exposed in the medical field etc.) have been taken as a basis for the mentioned extrapolations. Two difficulties must especially be taken into account in the assessment and evaluation of risks due to ionising radiation. One dilemma consists in the fact that radioactive radiation and radioactive substances can generally not be perceived by man. Another problem is that the effects, as far as they concern tumours and genetic damage, are of a stochastic nature and a relation between cause and effect cannot be immediately produced. A radiation exposure increases the probability of falling ill from cancer or leukaemia after a latency period of years or decades. It is not possible to differentiate these cases of illness from those illnesses originating from other causes. This is one of the reasons why the figures of persons damaged by the Chernobyl accident mentioned by different sources differ so much.

The Chernobyl Forum estimates that because of the Chernobyl accident in the former USSR between 5,000 and 10,000 additional fatalities could be caused by leukaemia and cancer. The following groups of persons were considered: 200,000 liquidators of the years 1986 – 1987, 120,000 evacuated persons from especially contaminated regions, 280,000 inhabitants of the highest-contaminated areas as well as about 7 million inhabitants of other radioactively contaminated zones in Belarus, the Ukraine and Russia.

In the current dispute about the number of victims it is essential to gain reliable information from the scientific area. Before long, BfS as an independent institution will therefore invite the authors of the different studies for an expert discussion. The objective is to elaborate and to evaluate the common state of knowledge, open technical questions and controversial interpreations.

Not only because of the radiation exposure but also because of the necessary disaster control measures and their consequences had the Chernobyl accident serious effects on the mental and intellectual health and the well-being of the population in the affected regions and the liquidators (action force). Stress as a result of relocations, rumours and misinformation about the radiation and loss of confidence in state institutions and their statements led to a “paralysing fatalism” with depressions, the impairment of self-confidence and psychosomatic diseases. The consequences of the enormous economic burdens of the affected countries as a result of the Chernobyl accident must be kept in mind as well.

Two cases must be basically differentiated in the biological effect of ionising radiation on man: the deterministic and the stochastic effect.

Deterministic radiation effects are radiation effects which only occur if relatively high dose threshold values (several Sievert) are exceeded. They can generally be directly attributed to a certain radiation exposure and occur at once or within a few weeks after irradiation. Deterministic radiation effects only become noticeable if a certain number of destroyed or damaged cells in an organ is exceeded and its function is thus impaired. Therefore, this type of damage only occurs above a minimum dose, the threshold value. The respective threshold doses vary for different deterministic radiation effects such as anaemia, loss of hair, etc. The haematopoietic organs, the mucous membranes of the gastrointestinal tract and the air passages as well as the gonads and embryonic tissue are especially radiosensitive.

Stochastic radiation effects, on the other hand, are radiation effects based on accidental, i. e. stochastic effects. If the information content of a cell is changed and in the following not sufficiently repaired by the organism and if such a changed cell remains viable and dividable the change can be passed on to future cell generations. Stochastic radiation effects occur in dependency of the dose with a certain probability. The latency period, i. e. the period between exposure and occurrence of the illness, can be years to decades. Depending on whether it is a germ cell or a body cell, it may be a change of the genes with possible health effects for future generations or malignant neoformations such as cancer and leukaemia may occur in the exposed persons themselves.

Radiation-caused cancer can only be determined with epidemiologic-statistical methods in relatively large person groups. However, they cannot be determined in individuals on the basis of the disease pattern. With regard to the disease pattern radiation-caused diseases do not differ from so-called spontaneous diseases.

On the basis of the level of radiation exposures determined in Germany due to the Chernobyl accident deterministic radiation damage can be excluded. Because of the low level of additional radiation exposure in Germany, which did not exceed 50 % of the annual natural radiation exposure of 2.1 mSv in the first year and, extrapolated to 50 years, will not exceed altogether 4 mSv, and against the background of the so-called spontaneous cancer frequency, it will hardly be possible to prove that radiation-caused cancer in Germany can be attributed to the Chernobyl accident..

The risk of dying of a radiation-caused cancer is about 1.2 % per 100 mSv. That means that, against the background of deaths caused by so-called spontaneous cancer of 2,000 to 2,500 persons out of 10,000 in Germany, a radiation exposure of 100 mSv results in about 120 calculated additional deaths due to cancer. The additional radiation exposure in Germany due to the Chernobyl accident of altogether no more than 4 mSv thus leads to a calculated additional number of deaths due to cancer which can only hardly if at all be distinguished from the general cancer death rate in Germany.

There are no indications for an increased occurrence of thyroid cancer in children in Germany either.

In a comprehensive investigation in mothers who were pregnant at the time of the reactor accident it was investigated if there was a higher number of pregnancies after the Chernobyl accident which ended unfavourably e. g. with premature births, deficiency births or still births in regions in Germany that were more strongly affected by the fallout. No damage in newborn children could be determined in this study. In a new BfS study the health of the children considered in the study on courses of pregnancy will now be determined.

Numerous other epidemiological studies in Europe outside the former USSR arrived at different conclusions with regard to infant deaths, frequency of malformations and tumours in children after the Chernobyl accident. Nearly all these studies were so-called ecological studies which compared the frequency of certain illnesses in regions with different contamination levels. The results from various investigations in different countries are contradictory in themselves. It cannot be excluded that the increased morbidity rates described in some studies are caused by accident or are due to other causes.

There has been no proof so far that in Germany or other countries of Central or Northern Europe the Chernobyl accident caused negative health effects due to radiation.

Radiation-caused cases of cancer can only be determined with epidemiological methods in relatively large person groups as a result of enhanced morbidity rates. They cannot be determined in individual persons because of the disease pattern. With respect to the disease pattern, radiation-caused diseases do not differ from so-called spontaneous diseases.

Most experts had not expected the observation that increased numbers of thyroid cancer occurred already four years after the reactor accident in the areas with the highest contamination levels in Belarus, Russia and the Ukraine. The thyroid cancer rate increased by the six-fold. Apparently, this consequence of the accident could directly be attributed to radiation exposure, since without radiation exposure thyroid cancer hardly occurs in children and youths. At least 9 children died from thyroid cancer. Most patients could be initially treated successfully by removing the thyroid and subsequent radioiodine therapy. The necessary aftertreatment must include regular examinations and the daily administration of medicaments to replace the functions of the thyroid, and leads to correspondingly aggravated living conditions.

In the liquidators, for whom doses above 150 mGy were documented, a 2.2-fold higher leukaemia incidence was determined for the first 10 years after the accident (1986 – 1996) in comparison with a group with lower exposures. This difference in the leukaemia incidence rates could not be confirmed any more for the second period of examination from 1997 to 2003, however.

An increase in cardiovascular diseases becomes apparent for liquidators who had received a dose above 150 mGy in less than 6 weeks in the 30-km zone. Except for the enhanced number of thyroid cancer and the enhanced breast cancer risk in women, no proof of increased cancer rates or leukaemia as a result of the Chernobyl accident could be furnished for children and adults of the population.

The Chernobyl accident led to the release of large amounts of radioactive substances into the atmosphere. These substances were distributed in the near field in Belarus, the Ukraine and Russia, and over the northern hemisphere, especially over Europe. The strongly changing meteorological conditions led to several radioactive clouds in different directions. The initial air flow transported the radioactive substances via Poland to Scandinavia, a second cloud drifted via Slovakia, the Czech Republic and Austria to Germany, and a third cloud finally reached the countries of Romania, Bulgaria, Greece and Turkey. The radioactive contamination in the affected areas resulting from this varied considerably, depending on the occurrence and level of precipitation during the drifting of the radioactive air masses. In Germany, the South was clearly stronger contaminated than the North, due to heavy local precipitations. In the Bavarian Forest and south of the river Danube up to 100,000 Becquerel (Bq) caesium-137 per square metre were locally deposited. In the North German Lowlands, the activity deposition of these radionuclides rarely amounted to more than 4,000 Bq/m2.

After the Chernobyl reactor accident, a multitude of radionuclides were deposited in Germany. Many of the mostly short-lived radionuclides have entirely decayed today. With regard to radiation exposure due to the reactor accident, practically only the long-lived caesium-137 plays a role today, of which only about 40 % have decayed so far because of its half-life of about 30 years. Part of it is bound to soil components, so that it has been withdrawn from the biological circle. In the areas of Southern Bavaria with the highest contamination levels, Cs-137 in the soil only contributes about 5 % additionally to the measured natural dose rate. Apart from the Chernobyl reactor accident, the radionuclides originating from the above-ground nuclear weapons tests also make a contribution to the radioactive burden in Germany.

Already in the summer of 1986, agricultural cultures sown or planted after the Chernobyl reactor accident were only contaminated with some Bq of radiocaesium per kg. Today the content of Cs-137 in agricultural products from domestic markets is also at this level and below. In Germany, about 100 Bq of Cs-137 per person and year are taken up with agricultural food.

The situation relating to food growing in forests is entirely different. In particular in mushrooms and game clearly enhanced Cs-137 activities can be measured even 20 years after the Chernobyl reactor accident. The caesium content of food growing in forests generally decreases only slowly. One can expect to find food growing in forests with higher levels of contamination in those parts of Germany which have especially been affected by the Chernobyl fallout, in particular the Bavarian Forest and the areas south of the river Danube. In other regions as e. g. in Northern Germany the activity values are correspondingly lower due to the fact that only little radiocaesium was deposited. Very high ranges of the Cs-137 content – also locally – in wild-growing mushrooms and game are characteristic.

Everyone wanting to keep radiation exposure for themselves as low as possible should therefore do without consuming mushrooms and game with relatively high contamination levels, especially of wild boar, e. g. from the Bavarian Forest.

On the mineral soils of many fields radiocaesium can be strongly bound to certain clay minerals. Thus, it can only be taken up by roots to a limited extent.

Forests are characterised by organic layers on the mineral soils. In these layers which are formed of decomposing mulch, caesium is easily available and is quickly taken up by soil organisms, mushrooms and plants. Caesium remains integrated in the very effective nutriment circles typical of ecosystems with low contents of nutriment and thus hardly migrates into the mineral soil layers, where it can be set by certain clay minerals as it happens on agricultural soils. The caesium-137 activity of edible mushrooms and game can thus be clearly enhanced and will only slowly decrease.

Generally, the Laender measuring bodies are competent for measurements of food. With measuring networks all over Germany, the Federal Government surveys the air, the radiation emitted by the soil, the federal waterways and the North Sea and Baltic Sea.